Gravity only begins to pull you down once you are in the air. When your feet are on the ground, resistance is equal to force, so Gravity is not still forcing you down. In your theory about Terra moving up, it never stops increasing speed, so instead of simply overcoming minimum gravity for the first second, you would need to overcome something moving at billions of miles per second, as its speed has been increasing for hundreds of millions of years (Or 6 thousand, according to some loonies). And watch an Olympic high-jumper on youtube. Could they really move that much faster than Terra for that long? Our muscles aren't that powerful. And for the airplane analogy, walking and jumping are two different things. If one were to jump on a plane, while it was accelerating, I daresay we would move backwards. Also, we can stand up on a plane because, for one, you are an attached object until you detach yourself, for instance by jumping. That means that we move at the same speed. Also, acceleration makes a big difference. If we were on a space shuttle accelerating directly upwards, we wouldn't be able to stand up.